Oximes isomeric

The magnitudes of the computed A and AG are relatively small for all four of the oxime isomerizations that have been considered. Thus the effects of intermolecular interactions with the surroundings—whether the pure liquid or solid phases or a solvent—may often determine which isomer is more stable in a condensed phase. (This point will be addressed again in Section VII.) For example, benzaldoxime is known to be in the anti form in an acidic environment but syn in a basic one . Temperature also plays a role in affecting K q. It should further be noted, as can be seen in Table 4, that even AE and AG in the neighborhood of just 3 kcalmol can produce Teq of two orders of magnitude. 

Lee and coworkers ", studying the Beckmann rearrangement of 1-indanone oxime derivatives 240, observed that the pure E and Z oximes isomerize under mild acidic conditions such as silica gel (equation 72). In the presence of Brpnsted or Lewis acids as silica gel or AICI3 the high rotational barrier of C=N double bond would be lowered by the formation of a complex between the tosylate and AICI3 241. This fact makes the... 

In contrast, the isomeric Erythromycin A 9(Z)-oxime 460 produced an isomeric nitril-ium ion intermediate 465 as the result of different oxime geomehy (equation 199). In comparison with the products obtained from the ii-oxime, isomeric lactam 468 and imino ethers (466 and 467) were isolated. Reaction conditions (including solvent polarity and the concentration of the reaction mixture) proved to have a strong influence on the yields of the various products of the rearrangement. 

Grigg and co-workers (310) recently examined the 1,3-APT reaction of various aldoximes (270) (R or R = H) with divinyl ketone (Scheme 1.56). While ketoximes 270 (R = R) form a mixture of adducts, 271 and 272 via nitrone 273, the aldoximes selectively afford 272 (as a mixture of endo and exo diastereoisomers). Under the thermal reaction conditions, the oxime starting materials can undergo ( /Z) isomerization, while the nitrone intermediate was expected to be unaffected and the isolated cycloadducts showed no interconversion via cycloreversion. Thus, the increasing selectivity for endo-212 [via ( )-273, R = H] over exo-212 [via (Z)-273, R = H] with the increasing size of the aldoxime substituent was attributed primarily to the inhibition of oxime isomerization by steric clash between R or R and the oxime OH. In contrast, Lewis acid catalysis, in particular by hafnium (iv) chloride, of the cycloaddition of various aldoximes with this dipolarophile gave exo-271 exclusively (216). 

Under conditions where oxime isomerization is facile, product distribution is clearly the result of a subtle combination of factors, including relative migratory rqititudes of substituents as well as die... 

The dioximes (and also monoximes) of benzoquinone, naphtha-quinone, and anthraquinone were iuvestigated by Elofson and Atkinson [97], who observed one wave in acidic and alkaline solutions and two waves in neutral solution the smallest wave, of kinetic character, was explained by nitrone — oxime isomerization at the electrode ... 

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